With the increasing demand of mobile communication users, the high data rate and high frequency spectrum efficiency is becoming one of the main requirements of the mobile communication system, and the advanced technology corresponding to it, such as OFDM technology and MIMO technology, also becomes the main stream supporting technology of the current broadband mobile communication system, and it is developed and applied in multiple practical systems, such as Worldwide Interoperability for Microwave Access (WiMax) system, Long Term Evolution (LTE) system, etc.
FIG. 1 is a schematic diagram of classical OFDM-MIMO transmitter; without loss of generality, take two transmitting antennas for example here. At the i-th symbol, data symbols X0[i,k], X1[i,k] (k represents the sequence number of sub-carrier) of signals in two paths are pre-coded firstly to obtain V0[i,k], V1[i,k]:
      (                                                      V              0                        ⁡                          [                              i                ,                k                            ]                                                                                      V              1                        ⁡                          [                              i                ,                k                            ]                                            )    =            W      ⁡              (                  i          ,          k                )              ⁢          (                                                                  X                0                            ⁡                              [                                  i                  ,                  k                                ]                                                                                                        X                1                            ⁡                              [                                  i                  ,                  k                                ]                                                        )      
W(i,k) is a 2×2 precoding matrix. In the open loop system, the function of precoding is generally mapping signals X0 and X1 in two paths to two different physical antennas, and obtaining a definite spatial diversity gain; in the close loop system, precoding can make radiation mode of the transmitting antenna further approximate to the intrinsic mode of MIMO channel by selection, so as to achieve the purpose of increasing the link gain. Whether it is open-loop or close-loop, transceiver can know precoding matrix W(i,k) on each sub-carrier either by reaching an agreement in advance or by way of channel associated signaling.
The data V0[i,k], V1[i,k] on which precoding is carried out multiplexes with the reference signal, which constitutes an OFDM data frame. For the signal of 0th path, as shown in FIG. 2, the reference signal R0 used for channel estimation is distributed in gap at a plurality of different OFDM sub-carriers. The grid marked with slashes and “R0” represents the time domain resources for transmitting the reference signal R0 in the signal of 0th path, and the content of signal is known by the receiver and transmitter; the grid with cross-lines corresponds to the time domain resources bearing the reference signal R1 and the antenna of 0th path does not transmit any signal in these time domain resources. Other places correspond to the transmission of data V0[i,k]. The reference signal R1 is transmitted in the signal of 1st path, and FIG. 3 also has the similar distribution of time domain, and the grid marked with slashes and “R1” in FIG. 3 represents the time domain resources for transmitting the reference signal R1 in the signal of 1st path while the grid with cross-lines correspond to the time domain resources bearing the reference signal R0 and the antenna of 1st path does not transmit any signal in these time domain resources. It should be noted that upload the time-frequency position of R0 in 0th path and don't transmit any signal in 1st path, which can avoid the influence of MIMO signal on the channel estimation. For the signal R1, there is similar design.
For the ith OFDM frame of signal of nth path, carry out IFFT transform, interpose cyclic prefix and send it to the nth antenna for transmission.
At the receiving end, there is a plurality of antennas to receive signal, and still take 2 antennas as an example, the classical OFDM-MIMO receiver is shown in FIG. 4. The mth antenna accomplishes synchronous frame segmentation, removing cyclic prefix and fast Fourier transform (FFT) in the OFDM demodulation module after receiving signals, so as to obtain the frequency domain signal of the multi-frame in the mth path.
The frame parsing module separates the received data signal part from the reference signal part according to protocol. The received signal module is:
                              (                                                                                          Y                    0                                    ⁡                                      [                                          i                      ,                      k                                        ]                                                                                                                                            Y                    1                                    ⁡                                      [                                          i                      ,                      k                                        ]                                                                                )                =                                            H              ⁡                              (                                  i                  ,                  k                                )                                      ⁢                          W              ⁡                              (                                  i                  ,                  k                                )                                      ⁢                          (                                                                                                                  X                        0                                            ⁡                                              [                                                  i                          ,                          k                                                ]                                                                                                                                                                                X                        1                                            ⁡                                              [                                                  i                          ,                          k                                                ]                                                                                                        )                                +                      U            ⁡                          (                              i                ,                k                            )                                                          (        1        )            
Wherein, Y0[i,k], Y1[i,k] represents the frequency domain signal at the kth data sub-carrier within the ith OFDM symbol received by the 0th and 1st antennas respectively, and H(i,k) is the 2×2 channel transmission matrix at the kth data sub-carrier within the ith OFDM symbol. U(i,k) is a 2×1 noise vector matrix here, and set noise power of each dimension as N0. k=0, 1, . . . , K, K is the largest sequence number of the sub-carrier, i=0, 1, . . . , I, and I is the largest sequence number of the OFDM symbol in one radio frame.
Using the received reference signal can obtain the channel estimation value from the nth transmitting antenna to mth receiving antenna, wherein the channel estimation value at kth sub-carrier within ith OFDM symbol is denoted as Ĥm,n[i,k], and combined with distribution pattern of the reference signal shown in FIG. 2 and FIG. 3, for the channel estimation of Ĥm,n[i,k], the method is generally as below:
Step A, carry out the initialization firstly, and set all the {tilde over (H)}m,n[i,k]=0, and {tilde over (H)}m,n[i,k] is the channel estimation value before filtering at lth sub-carrier within jth OFDM symbol;
Step B, perform the channel estimation for all of the sub-carriers bearing the reference signal within jth OFDM symbol:{tilde over (H)}m,n[j,l]=Rn*[j,l]Ym[j,l]
Wherein, Ym[j,l] represents the frequency domain signal at lth sub-carrier within jth OFDM symbol received from the mth antenna, Rn is the reference signal of the nth antenna transmitted at lth sub-carrier within jth OFDM, j is the sequence number of OFDM bearing the reference signal and l is the sequence number of the sub-carrier bearing the reference signal within jth OFDM.
“The reference signal sub-carrier” and “data sub-carrier” mentioned in the text are common names in the industry, which refers to the time domain resources bearing the reference signal, and the time domain resources are in the scope limited by one OFDM symbol and one sub-carrier, which can be called the Resource Element (RE), therefore the reference signal sub-carrier is also called the reference RE and the data sub-carrier is also called the data RE. Similarly, “sub-carrier” described in “sub-carrier of OFDM symbol” and “at the sub-carrier” mentioned in the text also refers to the time domain resources limited by one OFDM symbol and one sub-carrier, which is also called the RE.
Step C, for each sub-carrier which does not transmit any signal within jth OFDM, use the channel estimation value at two reference signal sub-carriers which have the same frequency domain as the sub-carrier and have most adjacent time domains to the sub-carrier to perform time domain interpolation, so as to obtain the channel estimation value at the sub-carrier;
basing on the distribution of the reference signal in FIG. 2 and FIG. 3, perform time domain interpolation at (l+3)th sub-carrier within jth OFDM:
                    H        ~                    m        ,        n              ⁡          [              j        ,                  l          +          3                    ]        =                    1        2            ⁢                                    H            ~                                m            ,            n                          ⁡                  [                                    j              +              4                        ,                          l              +              3                                ]                      +                  1        2            ⁢                                    H            ~                                m            ,            n                          ⁡                  [                                    j              -              4                        ,                          l              +              3                                ]                    
Of course, the interpolation operation of the step can also has other methods, for example, take channel estimation values at adjacent 4 or more reference signal sub-carriers to perform the time domain interpolation.
The above three steps obtain the channel estimation value at the reference signal sub-carrier of the signal in each path in the channel from nth transmitting antenna to mth receiving antenna.
Step D, calculate the channel estimation value at each data sub-carrier within jth OFDM symbol by the frequency domain interpolation, and perform frequency domain filtering for each sub-carrier, so as to obtain the channel estimation values after filtering at all of the sub-carriers within the OFDM symbol;
for each sub-carrier of jth OFDM symbol, perform operation by the following formula:
                    H        ^                    m        ,        n              ⁡          [              j        ,        k            ]        =            ∑              f        =                              -            L                    /          2                            L        /        2              ⁢                  ⁢                  F        ⁡                  (          f          )                    ⁢                                    H            ~                                m            ,            n                          ⁡                  [                      j            ,                          k              +              f                                ]                    
Wherein, L is the data window length of the frequency domain interpolation filter, which determines the range of value of f. F(f) is a low pass filter, and the design of its passband and stopband can refer to the priori knowledge of channel impulse response length and the like.
Step E, for each sub-carrier on OFDM symbol which does not bear the reference signal, use the channel estimation values at two sub-carriers which have the same frequency as the sub-carrier on two most adjacent OFDM symbols bearing the reference signal to perform the interpolation operation, so as to obtain the channel estimation value at the sub-carrier.
Performing the time domain interpolation at kth sub-carrier within (j+d)th (d=1, 2, 3) OFDM:
                    H        ^                    m        ,        n              ⁡          [                        j          +          d                ,        k            ]        =                              4          -          d                4            ⁢                                    H            ~                                m            ,            n                          ⁡                  [                      j            ,            k                    ]                      +                  d        4            ⁢                                    H            ~                                m            ,            n                          ⁡                  [                                    j              +              4                        ,            k                    ]                    
By the above operation of each step, the channel estimation values at whole time frequency resources of the channel from nth transmitting antenna to mth receiving antenna are obtained. When m,n takes different values, it may obtain Ĥm,n[i,k] of each channel, and these Ĥm,n[i,k] can constitute the estimated channel transmission matrix Ĥ(i,k).
The channel transmission model of formula (I) can be equivalent to:
                              (                                                                                          Y                    0                                    ⁡                                      [                                          i                      ,                      k                                        ]                                                                                                                                            Y                    1                                    ⁡                                      [                                          i                      ,                      k                                        ]                                                                                )                =                ⁢                                            H              ⁡                              (                                  i                  ,                  k                                )                                      ⁢                          W              ⁡                              (                                  i                  ,                  k                                )                                      ⁢                          (                                                                                                                  X                        0                                            ⁡                                              [                                                  i                          ,                          k                                                ]                                                                                                                                                                                X                        1                                            ⁡                                              [                                                  i                          ,                          k                                                ]                                                                                                        )                                +                      U            ⁡                          (                              i                ,                k                            )                                                              =                ⁢                                            Q              ⁡                              (                                  i                  ,                  k                                )                                      ⁢                          (                                                                                                                  X                        0                                            ⁡                                              [                                                  i                          ,                          k                                                ]                                                                                                                                                                                X                        1                                            ⁡                                              [                                                  i                          ,                          k                                                ]                                                                                                        )                                +                      U            ⁡                          (                              i                ,                k                            )                                          
Wherein Q(i,k)=H(i,k)W(i,k) is an equivalent channel.
Obtain the channel transmission matrix Ĥ(i,k), and know the precoding matrix W(i,k) used at kth sub-carrier within ith OFDM symbol, and the equivalent channel can be obtained according to the following formula:{circumflex over (Q)}(i,k)=Ĥ(i,k)W(i,k)
For kth sub-carrier within ith OFDM bearing data, it can solve the MIMO problem by using multiple methods, and it takes the common Linear Minimum Mean Square Error (LMMSE) method as an example.
      (                                                                      X                ^                            0                        ⁡                          [                              i                ,                k                            ]                                                                                                      X                ^                            1                        ⁡                          [                              i                ,                k                            ]                                            )    =            G      ⁡              [                  i          ,          k                ]              ⁢          (                                                                  Y                0                            ⁡                              [                                  i                  ,                  k                                ]                                                                                                        Y                1                            ⁡                              [                                  i                  ,                  k                                ]                                                        )      
Wherein the equalization matrix G[i,k] is obtained by the following formula:G[i,k]=({circumflex over (Q)}H(i,k){circumflex over (Q)}(i,k)+N0I)−1{circumflex over (Q)}H(i,k)
{circumflex over (Q)}H(i,k) is the conjugate transposed matrix of {circumflex over (Q)}(i,k), and by the above steps, it can finally obtain the estimation of X0[i,k] and X1[i,k].
The problem of the above receiver is that calculation amount is large, especially, the operation amount of solving equalization matrix is large. In the above algorithm, assume that the order of the frequency domain interpolation filter is 24 (L=24), solving MIMO problem at each sub-carrier within each OFDM symbol needs 155 real number multiplication, 141 real number addition and 8 division on average.
For the modern broadband high speed data system, in order to ensure higher throughput rate and better real-time, the above complicated algorithm needs a large number of calculations, and may result in more power consumption of receivers and higher cost of receivers.